Quantitative Raman spectroscopic study of the H2─CH4 gaseous system

Abstract

AbstractThe Raman spectra of pure H2 and CH4 gases and their 5 gaseous mixtures were collected at pressures from 1 to 40 MPa at ambient temperature. They were systematically analyzed in order to establish methodology for quantitative determination of composition and pressure of H2─CH4 bearing fluid inclusions. The peak positions of 4 vibrational bands of H2 in pure H2 gas decrease first and then increase after reaching their minima values at about 30 MPa. The wavenumbers of ʋ1 band of CH4 in pure CH4 gas decreases in wavenumber monotonically in the entire investigated pressure range. In H2─CH4 gaseous mixtures, both the peak positions of H2 and CH4 shift to lower wavenumbers with increasing pressure. The peak positions of ʋ1 band of CH4 can be used to determine the pressure of pure CH4 and CH4‐dominated fluids, where the peak position shifts are sufficiently large for precise pressure determination. The peak widths of H2 and CH4 in pure H2 and CH4 gases and H2─CH4 gaseous mixtures broaden as pressure increases. The peak area ratios and the peak height ratios between CH4 and H2 in H2─CH4 binary mixtures are sensitive to composition (i.e., molar ratio between CH4 and H2) but are almost independent of pressure. Thus, the peak area or peak height ratios can be used to determine the composition of H2─CH4 fluids. On the other hand, the peak height ratios of Q1(1) to Q1(3) bands in pure H2 gas or H2‐dominated gaseous mixtures are sensitive to pressure and insensitive to composition. Therefore, in H2‐dominated fluids, the relative peak heights of Q1(1) and Q1(3) of H2 is a better alternative for pressure determination. Based on our established Raman quantitative analysis method, we determined the composition and internal pressure of a natural H2─CH4‐bearing melt inclusion in quartz from Jiajika granite.